Red-shift
There is an observed increase in the wavelength of light from most
distant galaxies . The light is shifted towards the red end
of the spectrum, so the effect is called red-shift
Red-shift is seen by comparing the dark absorption lines in the
spectrum of a galaxy with the same lines from a source in the laboratory
(or the Sun): the galaxy's lines are all moved towards the red end
The wavelength increases because the galaxy is moving away from us
(receding). The faster it recedes, the bigger the red-shift
The further away the galaxies, the faster they are moving
and the bigger the observed increase in wavelength
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Red-shift the Sun (the reference) a nearby galaxy shifted this way a distant galaxy shifted this way blue end red end The dark ABSORPTION LINES are always the same set — they are made by the same elements. So if the whole pattern has moved, it is the light that has changed, not the elements. A galaxy moving AWAY from us has its light stretched to a longer wavelength — shifted towards the RED end. This is red-shift. The further away a galaxy is, the BIGGER its red-shift — so the faster it is moving away. Almost every galaxy is red-shifted, in every direction. That is the evidence that the whole Universe is EXPANDING, and it is why the Big Bang theory is accepted. More recent observations of supernovae show the expansion is speeding up, which is what 'dark energy' was proposed to explain. Three spectra one above the other with the same set of dark absorption lines: the Sun (reference), a nearby galaxy with the lines shifted slightly towards the red end, and a distant galaxy with the lines shifted further towards the red end
The rest of this topic is in the full Exam Guide The complete AQA GCSE Physics guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• Red-shift• Evidence for an expanding universe• The Big Bang theory• What is still not understoodRead the free physics topics →